Calcium pyrophosphate dihydrate crystals extracted from the synovial fluid of a patient with pseudogout viewed under polarised light microscopy. Calcium pyrophosphate dihydrate crystals extracted from the synovial fluid of a patient with pseudogout viewed under polarised light microscopy. Note the variation in size and morphology of the crystals, and the fact that some of the particles do not appear to exhibit birifringence. PAUL DIEPPE, and ANGELA SWAN Ann Rheum Dis 1999;58:261-263 ©1999 by BMJ Publishing Group Ltd and European League Against Rheumatism